Give Jenkinsfile greater control wrt its worker image.
Give Jenkinsfile greater control wrt its worker image.

Issues:
Fixes #791

Problem:
- We rely on a default worker image definition that is poorly understood
  because its definition lives outside the Jenkinsfile (in the
  bdo/jenkins-worker gitlab repo).

Analysis:
- Move the worker image definition into the Jenkinsfile.

Tests:
Ran the liberty unit-tests and 12.1.1-overcloud-vxlan Jenkins jobs and
verified the Jenkinsfile executed as expected.

richbrowne/f5-openstack-agent,richbrowne/f5-openstack-agent,F5Networks/f5-openstack-agent,richbrowne/f5-openstack-agent,F5Networks/f5-openstack-agent,F5Networks/f5-openstack-agent